This application responds to RFA-MH-14-060. The overall goals of this study are to test the effectiveness and examine the implementation of a peer-led healthy lifestyle intervention (Peer GLB) in supportive housing agencies serving diverse clients with serious mental illness (SMI) who are overweight or obese. The risk for obesity is twice as high in people with SMI compared to the general population. Minority origin contributes additional risk. Compared to non-Latino Whites with SMI, African Americans and Latinos with SMI are at higher risk of obesity and cardiovascular disease. The Group Lifestyle Balance (GLB) derived from the Diabetes Prevention Program is an established healthy lifestyle intervention that is efficacious in the general population for achieving clinically significant weiht loss (e5% weight loss). We aim to improve the reach of GLB in this study by: 1) bringing this intervention to people's doorsteps through its delivery in supportive housing agencies; 2) using peer specialists (former clients with lived experience of SMI) as facilitators to address economic and staffing demands; and 3) identifying implementation factors that can foster the use of this intervention in community settings. We propose to use a Hybrid Type 1 randomized controlled trial design to test the effectiveness of Peer GLB vs. Usual Care in supportive housing agencies and examine multi-level implementation factors. Peer GLB lasts 12 months and will be delivered by peer specialists in the housing agencies. Repeated assessments will be at baseline and at 6, 12, and 18 months post-randomization. To examine Peer GLB's implementation, we will conduct a mixed-methods study of contextual factors, collecting data from housing agencies' directors, program managers, staff, and peer specialists before the implementation of our trial and after recruitment for the trial has ended.